This invention relates to electronic measurement instruments and more particularly to apparatus and methods for calculating voltage or current in such instruments. Even more particularly, the invention relates to apparatus and methods for calculating RMS current and voltage signals using a plurality of filtration, rectification, and processing stages.
In order for a digital voltmeter to measure and display the correct RMS (Root-Means-Square) value of a signal, the signal needs to be converted from its AC form into a form suitable for a digital display. Three prior art methods have been used to accomplish RMS conversion, and none of the three methods can be used conveniently with CMOS (Complimentary-Metal-Oxide-Semiconductor) implementations, where the entire system is implemented in CMOS. The first method is thermal conversion which accomplishes the RMS conversion by passing the input signal to a device which is heated by the power in the signal, and since the temperature change will be directly proportional to the power or mean-square of the signal, this temperature change can be converted into the RMS voltage. This method usually involves using a matched heater pair which is part of a feedback system wherein the signal being tested is connected to one of the heaters and an instrument-generated DC signal is connected to the other heater. The system derives its error signal from the temperature difference of the two heaters. The DC signal required to null the temperature difference is equal to the RMS value of the unknown signal. In addition, in this type of system, the DC voltage must be converted to a digital signal with an analog-to-digital converter. Since CMOS integrated circuits cannot measure temperature change directly, many external components are needed to accomplish the thermal conversion method.
Another method used for the RMS conversion in prior art devices is to have a bipolar integrated circuit perform an analog squaring, averaging, and square root conversion. Since CMOS integrated circuits are more suited to digital functions, the analog squaring and square root converters cannot be placed in them, thus this method is not suitable for use with CMOS circuits.
A third method used in prior art devices for RMS conversion is the use of sampling techniques wherein the input signal is sampled at various times and the average of these sample values is computed. This would be a more complicated system involving some very high performance circuits which would not be suited to a modest performance, very low cost instrument.
It is thus apparent that there is a need in the art for an improved apparatus and method which will extract the RMS value from an unknown input signal. There is a further need for such apparatus and method that extracts such RMS value in such a way that it can be implemented in a CMOS process. There is a still further need in the art for such apparatus and method that is suited for a modest performance, very low cost instrument that could have other functions such as display control on the same CMOS chip.